If Jesus is God, then why did He say
the Father was greater than He?

"You heard that I said to you, ‘I go away, and I will come to you.’ If you loved Me, you would have rejoiced, because I go to the Father; for the Father is greater than I,'" (John 14:28).

     Jesus said the Father was greater than He not because Jesus is not God, but because Jesus was also a man and as a man he was in a lower position.   He was ". . . made for a little while lower than the angels . . ." (Heb. 2:9).  Also in Phil. 2:5-8, it says that Jesus "emptied Himself, taking the form of a bond-servant, and being made in the likeness of men . . ."  
     Jesus has two natures.  Jesus was not denying that He was God.  He was merely acknowledging the fact that He was also a man.  Jesus is both God and man.  As a man, he was in a lesser position than the Father.  He had added to Himself human nature (Col. 2:9).  He became a man to die for people.  
     A comparison can be found in the marriage relationship.    Biblically, a husband is greater in position and authority than his wife.  But, he is no different in nature and he is not better than she.  They share the same nature, being human, and they work together by love.
     So, Jesus was not denying that He was God.  He was simply acknowledging that He was also a man and as a man, he was subject to the laws of God so that He might redeem those who were under the law; namely, sinners (Gal. 4:4-5).

     For further reading please see the two natures of Jesus.

     SCRIPTURES QUOTED:

• Phil. 2:5-8, "Have this attitude in yourselves which was also in Christ Jesus, 6 who, although He existed in the form of God, did not regard equality with God a thing to be grasped, 7 but emptied Himself, taking the form of a bond-servant, and being made in the likeness of men. 8 And being found in appearance as a man, He humbled Himself by becoming obedient to the point of death, even death on a cross."
• Col. 2:9, "For in Him all the fullness of Deity dwells in bodily form,"
• Gal. 4:4-5, "But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, in order that He might redeem those who were under the Law, that we might receive the adoption as sons."
• Heb. 2:9, "But we do see Him who has been made for a little while lower than the angels, namely, Jesus, because of the suffering of death crowned with glory and honor, that by the grace of God He might taste death for everyone."